1. Field of the Invention
This invention generally relates to determining one or more properties of an insulating film. Certain embodiments relate to determining one or more properties of an insulating film that is imperfectly insulating.
2. Description of the Related Art
Fabricating semiconductor devices such as logic and memory devices may typically include processing a substrate such as a semiconductor wafer using a number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, insulating (or dielectric) films may be formed on multiple levels of a substrate using deposition processes such as chemical vapor deposition (“CVD”), physical vapor deposition (“PVD”), and atomic layer deposition (“ALD”). In addition, insulating films may be formed on multiple levels of a substrate using a thermal growth process. For example, a layer of silicon dioxide may be thermally grown on a substrate by heating the substrate to a temperature of greater than about 700° C. in an oxidizing ambient such as O2 or H2O. Such insulating films may electrically isolate conductive structures of a semiconductor device formed on the substrate.
Measuring and controlling such insulating films may be an important aspect of semiconductor device manufacturing. A number of techniques are presently available for making such measurements. For example, the physical thickness of such films may be measured with a profilometer or an atomic force microscope (“AFM”). Such techniques typically involve scanning a probe across a surface of the film on which a step is present. A measurement of the step height may be used to determine a thickness of the film. Such measurements may be disadvantageous because they require the presence of step in the film and are contacting in nature.
Electron microscopy techniques may also be used to determine a thickness of films such an insulating films. These techniques include, for example, transmission electron microscopy (“TEM”) and scanning electron microscopy (“SEM”). These techniques are generally destructive and are relatively expensive due to the ultra high vacuum equipment required for such techniques.
Optical techniques may frequently be used to determine optical parameters of insulating films. In such methods, incident light and reflected light may be measured as a function of incident angle, wavelength, polarization, and/or intensity. Using models related to the propagation of light through transparent materials, the optical thickness of such films may be determined. In addition, other optical parameters such as optical index of refraction, optical extinction coefficient, and reflectivity may be obtained. Such measurements may have the disadvantage of providing only optical information, which may not be perfectly related to electrical parameters of electrical devices built using these films. Furthermore, such optical methods may only be used to measure substantially transparent materials.
Electrical measurement techniques that rely on physical contact to a conductive electrode on top of an insulating film may be used to determine relevant electrical properties of insulating films using capacitance vs. voltage (“C-V”) and current vs. voltage (“I-V”) measurements. Such measurements have a long history and established utility. These measurements, however, may require a conductive electrode and a contacting probe. The necessity of direct physical electrical contact is particularly undesirable in many manufacturing situations.
Non-contacting electrical test techniques have been developed to provide electrical capacitance, electrical thickness, and electrical conductivity information about insulating films. Non-contacting electrical measurements of dielectric properties have a unique advantage of providing electrically derived information without the requirement of physical contact to an electrode on top of an insulating film. These techniques typically use an ion generation source such as a corona and a non-contacting voltage measurement sensor such as a Kelvin Probe or a Monroe Probe to determine electrical properties of the films. Examples of such techniques are illustrated in U.S. Pat. Nos. 5,485,091 to Verkuil, 6,097,196 to Verkuil et al., and 6,202,029 to Verkuil et al., which are incorporated by reference as if fully set forth herein.